New standards for microarrays
Traceable tests are an absolute necessity in many areas of science. The results must be comparable regardless of whether the test was carried out in laboratory A or B. However, an investigation by BAM Federal Institute for Materials Research and Testing showed that tests that use microarray technology do not always lead to the same results. Microarrays, also known as biochips, permit a number of different tests to be done on a coated glass plate and need an area of just a few square millimetres. "But appropriate standards and reference materials are not currently available," says Paul Dietrich of BAM. The problem is that the properties of these glass plates may influence the results. BAM's scientists have therefore investigated the different production steps of microarrays, carried out comparative tests and came to the conclusion that standards and possibly a model glass plate to be used as a reference are needed. This will help to better understand the tests with microarrays.
Currently, microarrays are used mainly in research, but their potential can significantly advance medicine as well. The test methodology is relatively inexpensive. "In addition, tests can create a high throughput, which saves time and requires only small amounts of initial material to be tested. A small drop of blood or a tiny piece of tissue is sufficient," says the chemist Dietrich. In the future, a rapid test at the bedside or at home would then be possible. Due to the lack of standards and controls, traceability of the results is often only tentatively possible, says Dietrich. But they are particularly necessary for medical use.
In addition to its own tests, BAM has also carried out a so-called interlaboratory comparison with five different laboratories. BAM cooperated with Scienion AG, a company specialising in microarray technology in a project sponsored by the German Federal Ministry of Economics and Technology (BMWi). In the round robin test the laboratories were provided with identically prepared glass plates as reference material, on which there was placed a small block of DNA. The technical term for this is described as 'printing' of the glass plates. For the test, instructions were provided, according to which the test had to be performed. "Despite identical microarrays, uniform protocols and reagents, the results obtained by renowned European institute's and clinic's laboratories differed widely from each other, and in some cases were far beyond the error tolerance limit," says Dietrich.
In the tests BAM used its own numerous surface analysis techniques: the surfaces of the glass plates and the boundary surfaces between the individual layers applied were examined using X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry. "With these techniques it is possible to look precisely at the first ten nanometres of a surface," says Dietrich. As a result of their investigations, the experts recommended the use of glass substrates that had been cleaned according to specific protocols. But what should you watch out for? The scientists can now give an answer
The structure of the biochip is usually very similar. The glass plate is first chemically coated so that a biomolecule such as a sugar, DNA, or protein component can bind to it. These serve as a capture molecule for the biomolecule to be detected, such as in a patient's serum or urine. The number of capture molecules located on the coated surface plays an important role in the significance of the tests. The same holds true for the storage of the biochip. These two aspects have also been investigated using spectrometric methods.
The project has now been completed. Further work will be carried out in a research project funded by the EU under the framework of the European Metrology Research Programme (EMRP) called "BioSurf". In addition to BAM, national metrology institutes, biochip manufacturers and users are participating in BioSurf. One project objective is to develop a reference material for a sugar-based microarray.